gps.protocol.BT.algorithm
Class BTAlgorithmPieceSelection

java.lang.Object
  gps.protocol.BT.algorithm.BTAlgorithmPieceSelection

public class BTAlgorithmPieceSelection
extends java.lang.Object

selects a suitable piece to request. BitTorrent uses strict priority as a first policy for piece selection, i.e., once a single sub-piece has been requested, the remaining sub-pieces from that particular piece are requested before sub-pieces from any other piece. This does a good job of getting complete pieces as quickly as possible. When selecting which piece to start downloading next, peers generally download pieces which the fewest of their own peers have first, a technique referred to as 'rarest first'. An exception to rarest first is when downloading starts. At that time, the peer has nothing to upload, so it's important to get a complete piece as quickly as possible. Rare pieces are generally only present on one peer, so they would be downloaded slower than pieces which are present on multiple peers for which it's possible to download sub-pieces from different places. For this reason, pieces to download are selected at random until the first complete piece is assembled, and then the strategy changes to rarest first. once all sub-pieces which a peer doesn't have are actively being requested, it sends requests for all sub-pieces to all peers. Cancels are set for sub-pieces which arrive to keep too much bandwidth from being wasted on redundant sends.

Version:

1.0, 5/10/2005

Author:

Weishuai Yang

Constructor Summary

BTAlgorithmPieceSelection(int n)
constructs a piece selection object

Method Summary

int	selectPiece(BTSession bts, BTPeer p) select a suitable piece for request

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

BTAlgorithmPieceSelection

public BTAlgorithmPieceSelection(int n)

constructs a piece selection object

Parameters:

n - seed for random object

Method Detail

selectPiece

public int selectPiece(BTSession bts,
                       BTPeer p)

select a suitable piece for request

Parameters:

bts - downloading session

p - bt peer from which to request

Returns:

selected piece index, negative if can't find one